Stabilization of 48- and 57-μ m CH3OD lasers for plasma diagnostics

Abstract

Simultaneously oscillated 48-μ m and 57-μ m CH3OD lasers pumped by a 9R(8) CO2 laser have been developed as optical sources of interferometer and polarimeter for fusion plasma diagnostics. The objective of this paper is to present a method for stabilizing the two-color far-infrared (FIR) laser outputs. A stable FIR laser requires a stable pump CO2 laser and a prevention of a back reflection from the FIR laser cavity. The frequency of the pump CO2 laser has been stabilized by an external Stark-cell modulation. The back reflection has been reduced by an optical isolator using a quarter-wave plate and an absorbing thin-film reflector. The two-color FIR laser outputs have been stabilized by controlling the cavity length. As a result of these measures, we have achieved output stabilities of ± 1.2%/h at the 48-μ m laser and ± 1.3%/h at the 57-μ m laser.